Air Traffic Services (ATS) have traditionally used dedicated point to point communications circuits to carry voice communications associated with the control of aircraft. In order to reduce leasing costs, the European ATS service providers (as represented, for example, by Eurocontrol) have defined a networking standard for ATS voice circuits which has been formalized in the Ecma International-European Association For Standardizing Information And Communication Systems' Standard ECMA-312 (Second Edition, June 2001) and subsequently adopted by the European Telecommunications Standards Institute (ETSI) as Standard EN 301 846. This ATS standard was also recommended by the International Civil Aviation Organization (ICAO) as a standard suitable for international use. This is addressed in ICAO Doc. 9084 AN/762, Manual on ATS Ground-Ground Voice Switching and Signaling (First Edition, 2002).
The ECMA-312 standard is based on the Integrated Services Digital Network (ISDN) PSS1 standard (also known as QSIG) and is adapted for use in Air Traffic Services. In the ATS environment, the ECMA-312 standard is known as ATS-QSIG. The physical interface defined in ECMA-312 is International Telecommunication Union —Telecommunication Standardization Sector (ITU-T) Recommendation G.703 Co-Directional Interface (CDI).
ATS-QSIG as defined in ECMA-312 requires the use of a 64 kb/s clear channel with octet integrity (D64U) as per ETSI specification ETS 300 290 (e.g. an E1 carrier channel). The ATS-QSIG 64 k/bs channel is mapped into four sub-channels of 16 kb/s each according to the mapping function ECMA-253 (ISO/IEC17310). Three of these sub-channels carry three voice signals encoded according to ITU-T Rec.G.728 at 16 kb/s. The fourth sub-channel, which also has an aggregate data rate of 16 kb/s, carries packet mode signaling information.
In general, 64 kb/s clear channels are not widely available in North America where 56 kb/s clear channels (e.g. a T1-carrier channel) are typically used. Where a 64 kb/s clear channel is available in North America, it carries a price premium and certain operational restrictions. This makes the use of ATS-QSIG unattractive for North American ATS providers. A similar situation exists in Japan where 56 kb/s clear channels are also typically used invention relates generally to.